Valves are commonly used to control the flow characteristics of a fluid. A conventional valve includes a valve body defining an inlet, an outlet, and a fluid flow path extending between the inlet and the outlet. A throttling element, such as a valve plug, is disposed in the path to control fluid flow. A stem connects the valve plug to an actuator, which moves the valve plug into and out of the flow path. The valve plug can be positioned between fully opened and fully closed positions to control the flow rate of the fluid through the control valve.
In such devices, a stem or other element is connected to the control element and extends from the valve body. The stem or other element is then connected to a typical actuator such as a spring and diaphragm actuator for movement of the stem, and thus the control element. The actuator typically receives control signals which dictate when the actuator moves the stem, as well as how much the stem is moved. While effective, the fact that an element extends from the housing allows for a passageway to exist between the fluid being regulated through the valve and the outside of the valve housing. It is therefore necessary to tightly seal the element against such leakage. This is particularly true when leakage of the fluid flowing through the valve presents a safety hazard or loss of the fluid is otherwise undesirable.
Accordingly, seals, typically referred to as packing, are integrated into the control valve to seal about the movable element or valve stem. In the case of a sliding stem valve, such packing is typically assembled from a stack of disks or washers. The packing is placed under constant load about the stem by a packing spring and packing flange with the packing flange being secured to the valve by fasteners or bolts. The packing spring supplies a constant spring force to load the packing, placing it in contact with the valve stem and thereby forming a fluid seal. The packing is generally comprised of an elastomer, graphite, or a combination of both. Continual operation of the valve causes degradation of the packing and an eventual failure of the fluid seal. It is therefore necessary to periodically change the packing to maintain the seal.
Certain difficulties however are encountered when removing the packing from such a sliding stem valve. The cylindrical opening around the valve stem is typically referred to as a packing box, and after prolonged usage, the packing provided within the packing box, particularly when used in a corrosive environment, becomes essentially a blind hole requiring the packing to be dug or otherwise physically extricated from the packing box. This in turn can cause damage to the stem or the packing box bore which will result either in less than an optimal seal being created when the new packing is inserted, or will require additional machining to make sure that the stem and packing box are of correct dimension and tolerance after removal. Alternatively, special tools can be designed for removing the packing, but again damage to the stem and bore can occur, and such special tools necessarily result in an added expense in maintaining the valve.